This invention relates to apparatus for controlling the movement of a movable element to bring the movable element to rest at a desired stopping position. More particularly, this invention relates to damping the final motion of the movable element when brought to rest at its desired stopping position.
Electrical and electronic damping systems for drive motors are well known. One such system utilized in a d-c servo motor control system for a serial impact printer is disclosed in U.S. Pat. Nos. 3,954,163 and 4,091,911. As disclosed in those patents, separate d-c servo motors are used to drive the carriage and print wheel of a serial impact printer. Each drive motor has a servo control system associated therewith which is of the so-called "dual-mode" type.
For example, and in the case of the drive motor for the print wheel, when it is desired to rotate the print wheel greater than a predetermined distance, the servo control system first operates in a velocity mode, where a signal indicative of the actual velocity of the print wheel is compared with a signal indicative of the desired velocity of the print wheel for the particular distance remaining to travel. The resultant error signal is supplied to the drive motor for controlling same. When the print wheel has ultimately been rotated to within said predetermined distance, the servo control system switches from the velocity mode to a linear mode, during which the actual velocity signal is compared with a sinusoidal position signal. As is conventional, the actual velocity signal serves to damp the controlled motion of the motor and thus print wheel in both modes of operation.
Other serial impact printers of the print wheel type currently on the market utilize open-loop stepper motor control systems, where there is no feedback of position or velocity information and no damping at or near the desired stopping position. These systems require relatively expensive and powerful stepper motors to maintain stability and accuracy of positioning at the required speed levels.
Closed-loop systems including "end-point damping" capabilities have been designed specifically for motors other than d-c servo motors, such as for multi-phase stepper motors and the like. While the presence of such closed-loop systems enables the use of cheaper and less powerful stepper motors, the closed-loop systems themselves have proven to be relatively complex and costly.
It would be desirable, therefore, to provide a motion damping apparatus for motors other than d-c servo motors, such as for multi-phase stepper motors, which is not as complex and costly as contemparary damping apparatus of this type.